1. Field of the Invention
The invention relates generally to gas control devices for controlling the fuel gas and oxidizing agent supply to a burner in an atomic absorption spectrometer and more particularly to gas control devices which are reproducibly adjustable.
2. Description of the Relevant Art
In an atomic absorption spectrometer, a line emitting light source emits a light beam, which comprises the resonant spectral lines of an element being looked for. This light beam passes through a flame from a burner and impinges upon a photoelectric detector. The liquid sample, which is to be analyzed, is sprayed into the flame by means of an atomizer. The sample is atomized by the flame and the elements present in the sample enter their atomic state. The attenuation of the light beam in the flame is indicative of the proportion of the element being looked for in the sample. The burner is operated with a fuel gas, for example acetylene, and air as the oxidizing agent. It is also known in the prior art to supply nitrous oxide gas (N.sub.2 O) as the oxidizing agent, instead of air, to the burner in order to obtain a hotter flame. Nitrous oxide has a higher proportion of oxygen than air and when it is used the supply of fuel gas is increased in order to provide the correct stoichiometric ratio of fuel gas and oxidizing agent.
In one type of prior art gas control device, needle valves are provided for the adjustment of the gas flows. The gas flows are indicated by means of a flow meter and adjusted by manual adjustment of the needle valves. In order to ensure maintenance of the gas flows once adjusted, a pressure regulator (or pressure reducer) is located upstream of each needle valve. These pressure regulators maintain constant pressure upstream of each needle valve. Thus, the gas flows are adjusted and regulated by means of adjustable restrictors at a constant inlet pressure.
Usually, the flame is first ignited with air as the oxidizing agent. The changing-over to nitrous oxide gas, if required, does not take place until after the flame is ignited. The increase in the fuel gas flow required, when operating with nitrous oxide, is obtained by opening a bypass to the needle valve.
In the above described type of prior art gas control device, the gas flows were adjusted by hand at the needle valves. Therefore, the gas control device had to be arranged so that the needle valves were easily accessible. This required, in many cases, relatively long conduit connections with the device.
A second type of adjustable prior art gas control device is described in copending application Ser. No. 704,830, U.S. Pat. No. 4,640,677, assigned to the same assignee to which this invention is assigned, in which the adjustment to gas and oxidizing agent flow is accomplished by control signals from an operating unit or a control unit. This copending application is hereby incorporated by reference.
According to the invention taught in the referenced copending application, the new and improved gas control device includes, in combination, a first restrictor and a first pressure regulator for the fuel gas line, and a second restrictor and a second pressure regulator for the oxidizing agent line, the regulators being connected upstream of the restrictors, respectively, and servomotors for reproducibly adjusting the pressure settings of the pressure regulators, respectively.
As a result, for adjusting the flow, the flow cross sectional area is not varied with constant pressure; rather the pressure is varied with a fixed restrictor. Consequently, the expensive needle valves required by the first mentioned type of prior art device are eliminated.
The use of a servomotor for the adjustment of the pressure regulator to a desired value permitted adjustment by control signals. Consequently, it was not necessary to make the restrictors easily accessible, as was the case with devices employing needle valves, which had to be adjustable by hand.
Also, since the pressure could be easily and reproducibly adjusted as desired, and each such pressure could be associated unambiguously with a certain flow, no additional flowmeters were required. The flow of the fuel gas could be increased in a well-defined manner by the servomotor, and the desired value of the pressure regulator would be readily obtainable, when changing over to a second oxidizing agent having a higher proportion of oxygen, such as, for example, nitrous oxide. A by-pass around the restrictor and control means as required by the first type of prior art devices could be omitted.
Although the device taught in the referenced copending application is indeed controllable and permits reproducible adjustments, particular situations make reproducible adjustment difficult. For example, if the atomizer nozzle has been displaced or the input prepressure has changed, the required variation to the servomotor control signals to account for these situations is not a factor known apriori.
The principal object of the present invention, therefore, is to provide a gas control device of the second type mentioned above, which permit reproducible adjustment of the gas flow rates even under unstable conditions such as input prepressure variation.
According to the invention this object is achieved by locating a flowmeter downstream of each pressure regulator and connecting each flowmeter to the control unit. In this way a feedback of the actual gas flow rate to the control unit is made and the adjustment of the pressure controls can be effected such that the desired gas flow rate is reproducibly adjusted.
Various flowmeters for measuring gas flow rates are know, e.g. suspension body type flow rate measuring devices and transducers. Such flowmeters however can be used only with difficulties in gas control devices working automatically. For example, the output of known, in suspension body type flow rate measuring devices, are not amenable to being evaluated directly by a control unit. Furthermore, the indicator of such suspension body type flow rate measuring devices is not sufficiently precise in the presence of elevated pressures as, for example, under the pressures typically required to operate an atomizer nozzle. Transducers are problematic because they supply analog output signals and therefore require additional A/D-converters.
To solve the aforesaid problems with known flowmeters, the preferred embodiment of the invention includes a flowmeter formed by a turbine wheel rotatably mounted in a housing. Signal generating means cooperating with the turbine wheel are provided to generate an output signal depending on the angular rate of the turbine wheel. The housing comprises a gas inlet and a gas outlet directed to the turbine wheel. The output signals of these flowmeters can immediately be supplied to the control unit and be evaluated. Thus the desired reproducibility of the adjustment of the gas flow rate for all operating conditions is achieved independent of, for example, new adjustments of the atomizer and changes of the prepressure applying to the pressure control.
There has thus been outlined rather broadly the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described more fully hereinafter. Those skilled in the art will appreciate that the conception on which this disclosure is based may readily be utilized as the basis of the designing of other apparatus for carrying out the various purposes of the invention. It is important, therefore, that this disclosure be regarded as including such equivalent apparatus as do not depart from the spirit and scope of the invention.
One embodiment of the invention has been chosen for purposes of illustration and description, and is shown in the accompanying Drawing forming a part of the specification.